C Language : Pre-Processor Directives

“C” Pre-Processor Directives: Pre-processors are executed first before executing other program statements by the compiler, like Header Files. Pre-processor statements are started with # symbol (Hash Sign). These are also called “C” Pre-processor directives. There are four types:

-         File Inclusion               (Header Files)

-         Macro Expansion                (Making Constants)

-         Conditional Compilation

-         Miscellaneous

File Inclusion: We have used multiple functions in our program. If these functions are Pre-Defined or Library Functions then we have to include some header files according to used functions in our programs. These header files are always included top of the program before all the functions and statements. We will use the keyword “include” to add a header file in the program.

Syntax:    # include<header-file>

Ex:           # include<stdio.h>

                # include<math.h>

Macro Expansion: In macro expansion, we will make a value as constant during execution of the program. We will use the keyword “define”. Use upper case letters only for assigning constant name. Always placed after header files at top of the program. You have to note that don’t use data-type, equal sign as assignment operator and semicolon as statement terminator.

Syntax:    # define CONSTATNT-NAME  value

Ex:           # define   PIE   3.14

                # define   DAYS   365

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// Find Area & Circumference of the Circle using Macro Expansion.                                       ME_CIR.C

# include<stdio.h>

# include<conio.h>

# define  PIE  3.14

void main( )

{

        int r ;

        float ar, cf;

        clrscr( );

        printf(“Input Radius of the Circle = “);

        scanf(“%d”, &r);

        ar=(float) PIE * r * r;

        cf=(float) 2 * PIE * r;

        printf(“\n Area = %f”, ar);

        printf(“\n Circumference = %f”, cf);

}

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